Curable organopolysiloxane compositions that cure by the hydrosilylation reaction are used as, for example, sealants—including protective coatings—for optical semiconductor elements in optical semiconductor devices such as photocouplers, light-emitting diode devices, solid-state imaging devices, and so forth. Since these optical semiconductor elements emit or receive light, these optical semiconductor element sealants—including protective coatings—must not absorb or scatter light.
As a consequence, Patent References 1 to 5 provide hydrosilylation reaction-curable organopolysiloxane compositions that, through the use of an organopolysiloxane having a high phenyl content, are able to form cured products that exhibit a high refractive index and a high optical transmittance.
However, the present inventors noted that these curable organopolysiloxane compositions had the following problems: their cured products do not always exhibit a high optical transmittance; they exhibit a poor durability of adhesion for the semiconductor element, lead frame, or package in contact with the composition during its cure, resulting in facile detachment; and their cured products yellow during long-term residence at high temperature, resulting in a decline in the optical transmittance. The present inventors also noted the problems of a poor durability of adhesion and an unsatisfactory reliability in the case of semiconductor devices provided with a semiconductor element that has been coated or sealed with these curable organopolysiloxane compositions and optical semiconductor devices provided with an optical semiconductor element that has been coated or sealed with these curable organopolysiloxane compositions.
A curable organopolysiloxane composition that solves the problem of a reduced durability of adhesion is disclosed in Patent Reference 6. However, the present inventors noted the following problem with this curable organopolysiloxane composition: because an essential component of this composition is a radical copolymer-type adhesion promoter, when an optical semiconductor element coated or sealed with a cured product from this composition is subjected to long-term use at high temperatures, this cured product undergoes yellowing and the optical transmittance declines.    [Patent Reference 1] JP 2003-128922 A    [Patent Reference 2] JP 2004-292807 A    [Patent Reference 3] JP 2005-105217 A    [Patent Reference 4] JP 2007-103494 A    [Patent Reference 5] JP 2008-001828 A    [Patent Reference 6] JP 2006-063092 A